Abstract
Rice in Rio Grande do Sul State is grown mostly under flooding, which induces a series of chemical, physical and biological changes in the root environment. These changes, combined with the presence of rice plants, affect the availability of exchangeable ammonium (NH4+) and pH of soil solution, whereas the dynamics of both variables can be influenced by soil salinity, a common problem in the coastal region. This study was conducted to evaluate the dynamics of exchangeable NH4+ and pH in the soil solution, and their relation in the solution of Albaqualf soils with different salinity levels, under rice. Four field experiments were conducted with soils with exchangeable Na percentage (ESP) of 5.6, 9.0, 21.2, and 32.7 %. Prior to flooding, soil solution collectors were installed at depths of 5, 10 and 20 cm. The soil solution was collected weekly, from 7 to 91 days after flooding (DAF), to analyze exchangeable NH4+ and pH in the samples. Plant tissue was sampled 77 DAF, to determine N uptake and estimate the contribution of other N forms to rice nutrition. The content of exchangeable NH4+ decreased over time at all sites and depths, with a more pronounced reduction in soils with lower salinity levels, reaching values close to zero. A possible contribution of non-exchangeable NH4+ forms and N from soil organic matter to rice nutrition was observed. Soil pH decreased with time in soils with ESP 5.6 and 9.0 %, being positively correlated with the decreasing NH4+ levels at these sites.
Highlights
Growing rice in Rio Grande do Sul is one of the main agricultural activities of the State
This study aimed to evaluate the NH4+ dynamics and pH and the relationship between them at different depths in soil solution with increasing salinity levels, under irrigated rice
Regardless of the soil salinity level and depth, there was a marked decrease in the levels of exchangeable NH4+ during rice growth (Figure 1)
Summary
Flooding, which can occur before (pre-germinated system) or up to 30 days after sowing (sowing on dry soil), promotes a series of physical, chemical and biological changes in the soil and rhizosphere. Changes in this environment are caused mainly by the lack of oxygen in the system, inducing a series of reductive processes in the soil. This dynamic is regulated by the presence and availability of electron acceptors and donors (organic matter) and is accompanied by changes in pH, redox potential (Eh), electrical conductivity (EC) and phenomena of sorptiondesorption and ion exchange, influencing the nutrient availability in soil and nutrient uptake by plants (Camargo et al, 1999). Losses of NH4+ may occur, mainly by diffusion from anaerobic to aerobic soil layers, since the presence of aerobic microorganisms in the topsoil favors nitrification and subsequent denitrification of N
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